Re: [PATCH V2] rs6000: Support to build constants by li/lis+oris/xoris

[Jiufu Guo <guojiufu@linux.ibm.com>]

Hi Segher,

在 11/28/22 10:18 PM, Segher Boessenkool 写道:
> On Mon, Nov 28, 2022 at 11:37:34AM +0800, Jiufu Guo wrote:
>> Segher Boessenkool <segher@kernel.crashing.org> writes:
>>> On Fri, Nov 25, 2022 at 04:11:49PM +0800, Kewen.Lin wrote:
>>>> on 2022/10/26 19:40, Jiufu Guo wrote:
>>>> for "li/lis + oris/xoris", I interpreted it into four combinations:
>>>>
>>>>    li + oris, lis + oris, li + xoris, lis + xoris.
>>>>
>>>> not sure just me interpreting like that, but the actual combinations
>>>> which this patch adopts are:
>>>>
>>>>    li + oris, li + xoris, lis + xoris.
>>>>
>>>> It's a bit off, but not a big deal, up to you to reword it or not.  :)
>>>
>>> The first two are obvious, but the last one is almost never a good idea,
>>> there usually are better ways to do the same.  I cannot even think of
>>> any case where this is best?  A lis;rl* is always prefered (it can
>>> optimise better, be combined with other insns).
>> I understant your point here.  The first two: 'li' for lowest 16bits,
>> 'oris/xoris' for next 16bits.
>>
>> While for 'lis + xoris', it may not obvious, because both 'lis' and
>> 'xoris' operates on 17-31bits.
>> 'lis + xoris' is for case "32(1) || 1(0) || 15(x) || 16(0)". xoris is
>> used to clean bit31.  This case seems hard to be supported by 'rlxx'.
> 
> Please put that in a separate patch?  First do a patch with just
> lis;x?oris.  They are unrelated and different in almost every way.
>
I just send out two patches, one for "lis; xoris" and one for "li; x?oris".

https://gcc.gnu.org/pipermail/gcc-patches/2022-December/607617.html
https://gcc.gnu.org/pipermail/gcc-patches/2022-December/607618.html

Maybe, do we prefer to separate into 3 patches for review easily :-)?

Thanks for review!


BR,
Jeff (Jiufu)

>> I hit to find this case when I analyze what kind of constants can be
>> build by two instructions. Checked the posssible combinations:
>> "addi/addis" + "neg/ori/../xoris/rldX/rlwX/../sradi/extswsli"(those
>> instructions which accept one register and one immediate).
>>
>> I also drafted the patch to use "li/lis+rlxx" to build constant.
>> https://gcc.gnu.org/pipermail/gcc-patches/2022-September/601276.html
>> https://gcc.gnu.org/pipermail/gcc-patches/2022-September/601277.html
> 
> Those seem to do many things in one patch as well :-(  It is very hard
> to review such things, it takes many hours each to do properly.
> 
> 
> Segher